This invention relates to phased array antenna systems, and particularly to such systems which are used for direction finding applications.
FIG. 1 illustrates a typical prior art phased array antenna system. Wave energy signals from a transmitter 11 are supplied to antenna elements by coupling network 13. The phase of signals supplied to each element 10, 12, 12', 14 14', 16, 16', 18, and 18' is nominally the same. Phase shifters 20, 22, 22', 24, 24', 26, 26', 28, and 28', each associated with one of the elements, are provided for varying the phase of wave energy signals, thereby to change the direction of the antenna beam radiated from the antenna. Since the antenna is fully reciprocal, transmitter 11 may be replaced with a receiver, and the phase shifters used to change the direction from which signals are received.
The phase shifters used in the antenna of FIG. 1 are typically digital phase shifters such as illustrated in FIG. 1A. The FIG. 1A phase shifter is a 3-bit phase shifter, which may typically be a diode or ferrite device. The phase shifter includes a bit 15 for changing input phase by 180.degree., bit 17 for changing phase by 90.degree., and bit 19 for changing phase by 45.degree.. Those familiar with such phased array antenna systems will understand that such digital phase shifters may have a larger or smaller number of bits, and that the bits are switched "on" or "off" by phase control signals to change the phase of supplied signals to approximate the desired phase. This approximation is more accurate if a larger number of "bits" are provided in the phase shifter.
FIG. 2 is a graph illustrating the ideal phase of wave energy signals to be supplied to the elements of the FIG. 1 array in order to steer the antenna beam to a selected radiation scan angle .theta., indicated in FIG. 1. For convenience, the required phase for each element is reference to the phase at central element 10, and plotted as a function of sine .theta. so that the phase functions are linear. It should be recognized that the phase values illustrated may be referenced to any particular phase value, or to the phase supplied to any particular element. The phase of element 10 has been selected as a reference phase merely for convenience.
Since the phase shifter of FIG. 1A cannot assume all values of phase change, in order to steer the antenna beam, it is necessary to set the phase bits 15, 17, and 19 to approximate the phase conditions illustrated in FIG. 2. FIG. 3 is a graph illustrating the phase of wave energy signals to be supplied to elements 14 and 14', which are symmetrically located in the array with respect to the array center. The graph illustrates only phase values for positive scan angles, and again, for convenience, phase values are plotted against the sine of the scan angle .theta.. The stepped lines in the graph illustrate the values which will be assumed by phase shifters 24 and 24' in order to approximate the required phase function at various antenna scan angles. From the graph, it is evident that the phase difference between the values of phase shifters 24 and 24' is not always the same as the ideal phase difference for perfect beam scanning. The difference between the ideal and actual phase difference is phase error .epsilon., which results in a pointing error in the radiated antenna beam. FIG. 4 is a graph illustrating the variation in the phase error for elements 14 and 14' as a function of the sine of the scan angle. This phase error has a maximum amplitude of .+-.45.degree. assuming 3-bit phase shifters. While it should be recognized that the presence of many elements in a phased array antenna tends to reduce the effect of this phase error, which arises from phase quantization, there will remain some inaccuracies in the steering direction of the array antenna as a result of the phase error in the phase difference between elements on opposite sides of the array center.
The antenna beam pointing error, which arises from phase quantization is relatively small and unimportant in many systems. In a high accuracy direction finding system, such as a microwave landing system or tracking radar, the phase quantization beam pointing error may be significant. It is also desirable to reduce phase quantization errors because the error may increase antenna sidelobes, an undesired effect in certain applications.
It is therefore an object of the present invention to provide an improved phased array antenna system having reduced phase quantization error.